1. Field of the Invention
The present invention generally relates to a method and apparatus for monitoring oil conditions in real time, and more particularly to a method and apparatus for monitoring oil by using certain parameters in order to determine an optimal timing for oil change.
2. Background of the Invention
As is commonly known, proper lubrication extends the service life of machines. When the machines are operated, a lubricant continues to be exposed to high temperatures, high speed, stress or loads, and an oxidizing environment. Thus, the lubricant tends to deteriorate and lose its lubricating effect. If the deteriorated lubricant continues to be used, then the heat becomes intensified and the lubricant is deteriorated at a faster pace. This eventually leads to significant damage or catastrophic failure of the machines.
Hydrocarbon base oils such as mineral oils and synthetic oils are chemically degraded through oxidative and thermal deterioration (thermal failure, compressive heating, etc.). Thus, chemical changes occur in the base oil molecules.
The oxidative deterioration of lubricant accelerates the depletion of antioxidant additives, thereby continuing to weaken its anti-oxidizing effect. This is worsened at the lubricating places of high temperature. In addition, this leads to the subsequent formation of corrosive acids, varnish and eventually oil-insoluble sludge. In order to prevent such occurrences, the lubricant must be properly treated or changed.
Unlike oxidation, thermal deterioration caused by heat or compressive heating is relatively not well understood. The thermal failure, which is the last step of the thermal deterioration, typically occurs when the base oil is directly exposed to hot surfaces or when there is a sudden and rapid increase of temperature associated with the adiabatic compression of entrained air bubbles in pumps and bearing, and other pressurized lubricating environments. This may cause a chemical change in the oil layer exposed to the hot surface of machine or the compressed air bubbles. Generally, the thermal failure of oils occurs at above 200° (400° F.). The thermal deterioration of oil, which causes varnish accumulation, has recently become one of the problems that occur over and over again with turbine lubricants.
Vanish may be formed from a variety of sources. In case of turbine and hydraulic oils, most varnish problems are caused through the thermal or oxidative deterioration of oil.
In order to perform an experimental evaluation of oil condition, an acid number test and a Fourier transform-infrared (FTIR) spectrometry analysis are used. In the acid number test according to ASTM D 664, a wet chemistry titration method is utilized to determine the concentration of acid present in oil. In addition, a standard FTIR analysis at laboratory is applied to the oil test. In case of the FTIR spectrometry analysis, a characteristic infrared peak at a wavenumber of 1740 cm−1 can be used as a criterion for determining oil oxidation.
Unlike oil oxidation, the thermal deterioration of oil typically occurs when oxygen is insufficient and forms a reaction by-product containing relatively less oxygen compared to the oxidation. As such, thermal deterioration does not change the acid number. When the thermal deterioration becomes a major cause for oil deterioration, molecules containing a carbon-oxygen double bond are not formed. Thus, it does not exhibit a peak at the wavenumber of 1740 cm−1 in the FTIR spectrum, which is used to evaluate the oxidative deterioration of base oil. However, a significant increase in the peak value in the FTIR nitration region (1600˜1640 cm−1 region of the spectrum) can indicate a thermal failure as a dominant mechanism of base oil deterioration.
However, the FTIR analysis and the total acid number (TAN) test cannot evaluate oil deterioration in a timely manner. Thus, they cannot be used for monitoring oil deterioration in real time.
U.S. Pat. No. 6,061,139 discloses a method and apparatus for monitoring thermal deterioration of a lubricant without interrupting the operation of equipment. This patent is based on the notion that the transmission loss of light, which occurs during passing through oil, is correlated with the thermal deterioration of oil. The diagnosis of lubricating oil deterioration is performed through the following steps.                a) Measuring a reference light intensity Io at a wavelength of 850 nm passing through an oil-free measuring cell having an optical length of t=1 mm.        b) Measuring a light intensity I passing through a measuring flow cell filled with test oil.        c) Computing a light transmittance loss by using equation        
      L    λ    =            -              (                  10          t                )              ⁢          log      ⁡              (                  I                      I            0                          )            
The measured light transmittance loss value of lubricant is compared with a threshold value and the result thereof is outputted in a display.
However, the above approach is disadvantageous in that the light transmittance loss is affected not only by the oil deterioration, but also by other factors such as moisture content, bubbles and particle contaminants.
One of the earliest indications for oil deterioration is a change in oil colors. Generally, the initial thermal failure causes a color change prior to the oxidative failure. Without any change in the acid number, viscosity or Fourier transform infrared oxidation (FT-IR-Ox) data of oil, a change in the oil color is a first indication for the thermal deterioration of oil. However, this means that the oxidation of oil has not yet occurred.
The change in oil color is caused mainly by carbon suspended in the oil and formation of oxidation-insoluble materials (chemical by-products caused by the failure of base oil). Oil color varies with the concentration and type of light-absorbing groups suspended in oil. These chromophore compounds are commonly referred to as color bodies.
Oil color is one of the parameters for the fresh oil specification. The color is defined according to ASTM D 1500, D1524 and D2129 standards. The ASTM has established a series of color standards ranging from colorless to dark brown. It has also assigned them numerical values ranging from 0.5 to 8.0 at intervals of 0.5. In this regard, the numerical value 8 refers to the darkest oil. These standards are made of colored glasses and oil samples can be compared side by side with the standard glasses. If a color has a value less than 0.5, then it is compared through another method using a series of platinum-cobalt standard solutions in a set of tall-form matched Nessler tubes. The platinum-cobalt scale runs from 5 to 300. These numbers denote the number of milligrams of platinum per one liter of the standard solution. The clarity of a sample is generally determined by using an oil sample, which is identical to one being used for color determination. That is, a light is focused on the sample and the signs for cloudiness, sludge or particulate matters are identified.
ASTM D 1500, D 1524 and D 2129 standards are useful in conducting laboratory oil tests. However, they are not useful for performing real-time oil monitoring. Therefore, there is a need for a new lubricant testing procedure in order to predict oil deteriorations in a timely manner.
In addition to the chemical deterioration of oil, total oil contamination indicating the physical contamination of oil is of great importance in oil performance. The oil contamination typically results from the chemical deterioration of oil and the influence of mechanical particles and water/bubble contents upon the performance of machines. The mechanical particles may originate from the system (internally or externally). The internal source includes rust, wear, sealing products, etc. The external source includes dust, welding spatter, metallic debris, etc., which can be introduced into the system through ineffective seals, unclean oil fill pipes or unclean make-up oil. Moreover, the used oil often contains water and bubbles, which originate from the outside of a system.
The total contamination of oil can be assessed by the optical density and turbidity of the oil.
U.S. Pat. No. 6,151,108 discloses a technique for measuring the contamination of lubricating oil in real time. This technique allows total contamination to be distinguished from contamination by ferrous wear particles. The total oil contamination is defined by a difference in the optical densities of fresh oil and used oil. The content of ferrous particles is measured by a change in the optical density of a test oil sample under the influence of magnetic field.
U.S. Pat. No. 6,937,332 discloses an evaluation technique of total oil contamination, which is applied to an oil quality sensor based on turbidity measurement. According to the above technique for determining oil quality, light is transmitted from a light source into a flowing liquid through a flow tube. Further, the quantity of transmitted light, which is represented by the transmitted light path, is measured by a first light sensor. The amount of perpendicular light-scattering is measured by a second light sensor, while the amount of backward light-scattering is measured by a third light sensor. The turbidity of the fluid is determined based on the measured amounts of the transmitted light, perpendicular light-scattering and backward light-scattering. It is then used for determining the quality of the fluid. The sensor can measure the total contamination of oil and the oil contamination by water and antifreeze (ethylene glycol).
The techniques disclosed in U.S. Pat. Nos. 6,151,108 and 6,937,332 may provide information on the physical state of oil. However, they cannot provide information on the chemical deterioration of oil, especially oxidative and thermal deteriorations of oil.
Therefore, the present invention seeks to resolve the above problems of the conventional technologies. In this regard, it is an object of the present invention to provide a method and apparatus for simultaneously monitoring oxidation and the thermal deterioration of oil, i.e., chemical deterioration.
Another object of the present invention is to provide a method and apparatus for monitoring oil, wherein the chemical deterioration and the total contamination of oil are simultaneously computed from measured data. Further, the level of oil deterioration based on the computation is monitored in real time.
A still yet another object of the present invention is to provide an apparatus for monitoring oil deterioration in real time, which has a simplified and compact structure, thereby being mountable on every single machine to be monitored.